Sulfur has a number of allotropic forms, including orthorhombic, amorphous and monoclinic forms, with specific gravities of 2.07, 2.046, and 1.96 Mg/m3, respectively. Elemental (unmodified) sulfur undergoes a complex transition in two steps between allotropic forms, from liquid sulfur above the melting point at 119.2° C. to solid sulfur at room temperature (or below 95.5° C.). Upon solidification sulfur initially takes a monoclinic β-phase. It undergoes 7% contraction in volume compared with liquid sulfur. If elemental sulfur is used as a binder with mineral aggregates to form a sulfur concrete material, this contraction leads to sub-pressure in pores and on surfaces.
The tensile capacity of sulfur, which is only 0.3-0.4 MPa, is not capable of enduring the strain, and micro-cracking is inevitable. This opens up the elemental sulfur concrete material somewhat to moisture penetration.
On further cooling of the sulfur, the monoclinic (β-phase) transforms into the stable orthorhombic form (α-phase), at 95.5° C. This transition is rather rapid (less than 24 hours) and leads to a further decrease of volume by 6%. It causes strain on the binder and cracking within the material, whether volume compensation has been made at solidification or not. Historically, elemental sulfur concretes have failed (in the mechanical sense, due to disintegration) when exposed to humid conditions, repeated cycles of freezing and thawing and immersion in water.
In principle, there are two ways of treating this problem, relieving the material from imposed stress due to contraction; either by modifying the sulfur binder in such a way that it stays for a long time in the β-phase (the chemical way) or accepting the transition into the α-phase but preventing, at least for a long time, the sulfur binder from forming micro sulfur crystals which would cause contraction (the physical way). This is explained further in e.g. U.S. Pat. No. 4,293,463.
The chemical way is to combine sulfur with a modifying agent that chemically modifies the sulfur in order to inhibit transformation to the orthorhombic structure. Suitable substances that may be used for this include dicyclopentadiene, or a combination of dicyclopentadiene, cyclopentadiene and dipentene.
The physical way is to combine sulfur with a modifying agent that physically modifies the sulfur. Typically the modifying agent is an organic plasticizer. Usually it comprises a polymer such as an olefin hydrocarbon polymer (e.g. RP220 or RP020 produced by Exxon Chemical or Escopol).
A durable sulfur concrete material not only requires a stable binder but also a composition of aggregates and binder such that the full composite remains stable and durable (e.g. it has limited absorption) under fluctuating temperature and moisture conditions.
Aggregates have been the focus of many efforts in seeking a durable sulfur concrete product. For example, moisture absorption can be limited by the use of dense graded mineral aggregates, and proper composition design with binder, mixing and consolidation. The selection of different aggregates, which will be appropriate for each particular application, is necessary for a sulfur concrete material. To meet the requirement of durability, cleanliness and limits of harmful substances, the composite aggregates must meet the ASTM C 33 specifications according to the ACI Committee 548. To determine an aggregate's suitability for a particular use, it is recommended that preliminary testing be carried out for verification.
Corrosion resistant aggregates must be clean, hard, tough, strong, durable and free of swelling constituents. They should also resist chemical attacks and moisture absorption from exposure to acid and salt solutions. Moisture absorption and dissolution losses should not exceed 1% in a 24 hour period.
When clay is contained within solidified sulfur concrete, the clay is believed to have an absorptive capacity, which will allow water to permeate through the material. When clay absorbs water, expansion occurs, resulting in deterioration of the product. Thus, clay-containing aggregates should not be used in producing sulfur concrete without treatment for limiting the swelling capacity.
Sulfur concrete is prepared in a different way from Portland cement concrete. New gradation designs have been developed based on the technology for asphalt concrete. The intention was to develop aggregate mixtures with maximum density and minimum voids in the mineral aggregate, so that less sulfur is needed to fill the voids of the mixture. The optimum range for the sulfur content of the sulfur concrete is slightly less than the amount necessary to fill the aggregate to 100% saturation, yet high enough to keep the final void content less than 8%. This, in most cases, results in higher strength materials, because improved aggregate contact means less shrinkage after solidification.
The mineral filler forms, with the binder, the paste which coats and binds the coarse and fine aggregate particles to produce a strong and dense product. Fillers should (1) control the viscosity of the fluid sulfur-filler paste, workability and bleeding of the hot plastic concrete; (2) provide nucleation sites for crystal formation and growth in the paste and minimize the growth of large needle-like crystals; (3) fill voids in the mineral aggregate, which would otherwise be filled with sulfur, reducing hardening shrinkage and the coefficient of thermal expansion; and (4) act as a reinforcing agent in the matrix to increase the strength of the formation.
Therefore, to meet the above mentioned functions, the filler must be reasonably dense-graded and possibly finely divided, so as to provide a large number of particles per unit weight, especially to meet the function (2) as described above (provision of nucleation sites).
As the preceding discussion indicates, much research has focussed on physically controlling the adverse effects of sulfur concrete by controlling the aggregates. Such physically controlled materials are not always available, for instance in arid lands.
Various uses have previously been suggested for sulfur concretes, including commercial applications such as the construction of chemical vats, the encapsulation of radioactive waste or mixed wastes in sewage and brine handling systems, and electrolytic baths. Sulfur concretes have also been used by the Corps of Engineers in repairing dams, canal locks, and highways. The use of sulfur concrete materials as barrier systems has been accepted by the US Environmental Protection Agency.
Various uses have also previously been suggested for modified sulfur concretes, including rigid concretes, flexible paving, spray coating, grouts and the temporary containment of corrosive compounds such as acidic and salt solutions.
However, it has not been suggested to use such modified sulfur concretes to restrict permeation over a long time period. The restriction of permeation over a long time period may be useful in, for instance, waste containment. Thus, hazardous waste requiring long-term containment calls for a containment construction comprising a barrier that restricts permeation over a long time period. In this instance, the barrier can help to protect subsurface soils and groundwater from contamination by toxic substances in the hazardous material due to leaching and movement by ground water action. It can also provide a means for isolation and confinement of the toxic substances within their storage or disposal host environment.
Materials that are currently being used for this purpose include materials that have mainly been used in engineering practice such as hydraulic cement, clay based soil, thermoplastic organic binders and thermosetting organic binders. These materials are being utilized as containment barrier systems around hazardous materials being stored or disposed of in underground or surface excavations. Clay based soil barriers are generally used because of their low hydraulic conductivity. In arid land regions, where the clay materials are unavailable, prefabricated synthetic materials in combination with bentonite are generally used. It has been proposed to use ordinary and special cements and concretes but this approach has not proven entirely satisfactory.
Among the possible desirable properties for a barrier are the following. It should (1) form an impervious barrier to the action of ground and saline waters; (2) have a low leaching rate, particularly by ground or saline waters; (3) be relatively inert; (4) have good resistance to chemical and physical degradation and biological processes; (5) be compatible with the containment construction and any uncontained hazardous material in the host environment; (6) exhibit a long-term satisfactory behaviour as a barrier or backfill material in the storage or disposal environment; (7) be in plentiful supply and at a reasonable cost; and (8) be easy to handle and control from an operating and manufacturing point of view. Materials suitable for use as barriers for hazardous waste require hydraulic conductivity in the order of 10−9 m/s or less.
Some researchers have used sulfur to solidify liquid low-level radioactive waste. The solidified material is disposed in a landfill which uses e.g. clay based barrier systems and geosynthetics. Thus, the leaching of metals from the sulfur-based material has not been a major concern. In this scenario, one would expect that metals will be leached out from the sulfur matrix but be contained by the barrier system. In most cases the sulfur matrix has been prepared from molten sulfur without any chemical additives.
When researchers have attempted to use chemical additives for sulfur modification, durability of the sulfur concrete has been questionable because of the type of chemicals used. Long-term durability to chemical attacks and temperature has been examined but the necessary level of satisfaction for engineering applications has not been met.